Piston



FIG. 1 is a top perspective view of a piston showing our new design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a left side view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a top view thereof; and,

FIG. 7 is a bottom view thereof.

The broken lines in the Figures show portions of the piston that form no part of the claimed design. 

CLAIM The ornamental design for a piston, as shown and described. 